1. Field of the Invention
The present invention relates to a liquid discharge head. More specifically, the present invention relates to a liquid discharge head that discharges a liquid supplied from a flow path through which ink circulates, to print out an image.
2. Description of the Related Art
It is known that the following problems arise in discharging a liquid from a liquid discharge head, when ink thickening occurs near a discharge port, if quiescent time in which no image is printed out is longer than predetermined time.
(1) Color unevenness of the image due to a change in a discharge amount.
(2) Deterioration in impact precision due to a change in discharge velocity.
(3) Non-discharge in which the ink is not discharged. Causes of these problems are that a meniscus surface of the ink present near the discharge port contacts external air, and volatile components contained in the ink evaporate, resulting in the ink thickening.
In particular, if the quiescent time is long, then viscosity conspicuously increases and solid components of the ink adheres to an area in the neighborhood of the discharge port. The solid components increase a liquid resistance of the ink. If the viscosity further increases, discharge failure occurs.
As one of measures against such an ink thickening phenomenon, a method is known for causing ink supplied to a recording head to circulate through a circulation path as discussed in Japanese Patent Application Laid-Open No. 2006-88493. The ink is introduced into the discharge port from an upstream part of the circulation path, the introduced ink flows to a downstream part of the circulation path, and the ink is discharged while the ink is circulating. The following technique is also known as discussed in Japanese Patent Application Laid-Open No. 7-164640. According to the technique, common liquid chambers independent of each other are provided for supplying ink from two directions, and a pressure difference is generated between the common liquid chambers, thereby generating a circulatory flow.
However, the inventor discovered that these conventional techniques have the following problems if the ink is discharged during circulation.
With a configuration of each of the conventional techniques, if the ink is discharged during the circulation, then a discharge direction is inclined to change an impact position and image degradation often occurs. Furthermore, even if a main drop discharged from the liquid discharge head impacts on a predetermined position without receiving the influence of the circulation, a discharge direction of sub drops (satellite drops) accompanying the main drop is inclined and impact positions of the satellite drops often change.
The reason for this phenomenon will be described with reference to FIGS. 3A to 3D. In FIGS. 3A to 3D, a liquid flow path 11 is formed to be symmetric about a discharge port 12 and an energy generating element 13. Since a circulatory flow 14 in the liquid flow path 11 is a one-directional flow, this circulatory flow 14 is asymmetric about the discharge port 12. Accordingly, a pressure difference is generated between an upstream side into which the circulatory flow 14 is introduced and a downstream side from which the circulatory flow 14 is discharged, near the discharge port 12. As a result, a meniscus surface 17 formed on the discharge port 12 is asymmetric between the upstream side and the downstream side, a discharge direction is inclined, and an impact position changes (see FIGS. 3C and 3D). This influences an image to be printed out.